Among the events of fertilization, few are more important, yet enigmatic, than interactions between the sperm and egg membranes. Although several sperm proteins that bind to mammalian oocytes have been identified, there has been less success in identifying oolemmal receptors for sperm ligands. The unique testis-specific c lysozyme-like, intra-acrosomal protein SLLP1, was reported to lack bacteriolytic activity (1); localize to mouse sperm acrosomal membranes, and have oolemma binding properties (2). SLLP1 antibody and recombinant (r) SLLP1 were noted to block fertilization and sperm-egg binding in mice, suggesting that the protein may play a role in sperm/egg adhesion.
Molecules posited to be involved in sperm-oolemmal binding and fusion include the ADAM family ligands and their oocyte integrin receptors (3-6). However, gene targeting studies have demonstrated that the sperm ADAMs including fertilin a (ADAM1), fertilinβ (ADAM2) and cyritestin (ADAM3) are important primarily for the process of zona pellucida binding rather than for gamete fusion (7-9). Attention has also focused on tetraspanins (e.g., CD9, CD81), on GPI-anchored proteins, and on PIG-A which are expressed on oocytes. Data suggest that these proteins are important for the sperm-oocyte fusion step, but not for the binding process (10-12). Although CD9−/− female mice produced eggs that matured normally, sperm-egg fusion failed in these animals (13, 14). Targeted disruption of CD81 resulted in a 40% reduction in fertility of only female mice while mice lacking both CD9 and CD81 were completely infertile, indicating their complementary roles in sperm-egg fusion (15). It is noteworthy that sperm ligands that interact with oolemmal tetraspanins have not been identified. Several sperm membrane ligands have been implicated in fusion with the oocyte although their oolemmal receptors are unknown. Epididymal protein DE (CRISP1) has been implicated in sperm-oocyte fusion (16) and a specific binding region within CRISP1 was mapped, however CRISP1 knockout male and female mice showed no differences in fertility compared to controls (17). Recently, Izumo, an Ig-domain molecule localized within the acrosome was shown to be essential for sperm-egg fusion (18) although its oolemmal receptor is still unknown.
Moreover, there has been a concerted effort to identify biomarkers and differentiation antigens that are specific to the sperm or the egg in order to target these cells for contraceptive purposes, including drug and vaccine development. These approaches have included monoclonal antibodies directed a the gametes, proteomic, transcriptomic and genomic approaches (Nass et al., Nat Rev Drug Discov. 2004 October; 3(10):885-90); Nass et al., Science. 2004 Mar. 19; 303(5665):1769-71; Nass et al., National Academy Press, p 27-77, (2004), Contraception. 2008 October; 78 (4 Suppl):S28-35. Epub 2008 Aug. 22; Aitken et al., Contraception. 2008 October; 78 (4 Suppl):S18-22. Epub 2008 Jun. 12.)
Candidates proteins for contraceptive targeting must meet selective criteria including 1) restriction of the protein to the gamete; 2) an essential role for the protein in key stages of gametogenesis, fertilization or implantation; 3) accessibility of the drug target at the cell surface or within a select window of differentiation; 4) structural domains amenable to drug targeting (drugability); and 5) the restriction of the target to selected stages of gamete differentiation that permit targeted drug action and contraceptive reversibility in the case of human applications. Until now, few proteins have been elucidated that meet all of these criteria.
There is a long felt need in the art to identify both sperm and oolemma specific interacting proteins involved in the process of fertilization and to find methods to regulate these interactions to regulate fertility and contraception. The present invention satisfies these needs.